Certain vehicle designs, such as hybrid electric vehicles (HEV), are able to selectively utilize different energy sources in order to optimize fuel efficiency. An HEV having a full hybrid powertrain can use either or both of an internal combustion engine and a high-voltage energy storage system (ESS) for propulsion. That is, a typical full HEV can be electrically propelled, usually immediately upon starting the HEV and during vehicle speeds up to a relatively low threshold speed. One or more high-voltage motor/generator units (MGU) may alternately draw power from and deliver power to the ESS as needed. Above the threshold speed, the engine can be started and engaged with a transmission to provide the required propulsive torque.
By way of contrast, the powertrain of a mild HEV typically lacks the capability of propelling the HEV via purely electrical means, but nevertheless retains certain key design features of the full HEV, e.g., the regenerative braking capability used for recharging the ESS using the MGU, as well as the capability of selectively shutting down or powering off the engine. The capability of an HEV to selectively shut off and restart the engine when the vehicle is at a standstill, and/or when operating in a stabilized low-speed drive mode, is of particular fuel-saving benefit relative to conventional idling vehicle designs.